Recently I was reading a comic, in it the group of heroes could take some drugs to turn them into anthropomorphism of insect temporarily. It gave me some idea of building up a character that can turn into insect-humanoid to fight for a few rounds. When I looked up through the rules, I found most abilities were about anthropomorphism of animals such as beastmorphs / anthropomorphic animal.

Then, I start to think about the most (none scientific) fantasy works I've read. Animal-headed humanoids seem to be pretty common, human-headed animals are rare but still can be seen once for a while. Insect anthropomorphism, however, just seems to be extremely rare. The closest thing is drider but spiders aren't insect. There are some anthropomorphism of spiders and centipedes in classical literature like journey to the west but neither of them are insect as well.

Is it some kind of taboo to use it in none-scientific fantasy settings?

The further something is biologically away from us, the harder it is for us to anthropomorphize. It's easy for us to anthropomorphize dogs, cats, etc, because we see traits in those animals that remind us of ourselves. Much harder to do the same with something like a beetle or other invert, and then it's more likely to make us feel more revulsion about it than anything else.

I have a semi-humanoid insectoid race in my homebrew called Entomorphs. They have basic bipedal humanoid shape - six limbs but two are legs (digitigrade, but distinctly legs) while the other two are arms (though the secondary pair is unsuited for fighting or casting with), semihumanoid torso (females tend to be curved and bulkier in shape, despite lacking breasts, compared to the more slender males), etc. - but are otherwise moth-like in feature. A few species have unusual traits, such as their legs being grasshopper-like (awesome jumpers), gaining a scorpion tail instead of a full thorax (sting attack), extra eyes (they normally have two humanoid-shaped dominant compound eyes, but some will have extra smaller ones), and so on.

More accurately, I have a species of magical insect called the Entonyd. They are a symbiotic species that bond with a host, cocoon them, and metamorphose them into Entomorphs. Normally they bond with animals, and don't live long as a result, but occasionally they do so with humanoids (regardless of the original species the resulting Entomorph is little different from the rest), and these True Entomorphs tend to live much longer. Once transformed, though, they're a self-sustaining species - Entomorph offspring are born from eggs, already transformed, and go through a larval stage that lasts about a year. They hatch as nymphs (wingless, poisonless for those that have bites or stings, and their exoskeleton is weaker and thus their ensuing natural armor bonus is nonexistent) then grow into adulthood in about ten to fifteen years.

Ant people are the most common in fantasy -- see the fomori in D&D and whatever the heck the ant-people are called in Pathfinder.

And then if you think about it, even though they are largely humanoid look, fairies with butterfly and other insect wings are common. And I think in general, you will see hybrids of animals and humanoids that are more chimeric than a crossbreed--see driders for another example.

Also the fae in Pan's Labrynth, some of them had a very insectile feel to them.

I just thought of a creepy way to introduce a race of insect people.

The party is trudging through the forest when one of the insect people steps out and asks them what they are doing. If their intent is peaceful he makes a whistling noise and the leaves and bark and rocks of the forest come alive as the insect people step out from the safety of their camoflauge. Then the party realizes they had been surrounded.

A lot can be done with "hives" and "queens" also.

They might have a racial fear of the Tengu, since birds prey on insects so much.

Incecto-morphism (in which I would personally include spiders, millipedes and other "bugs") are indeed rarer but not nonexistent. In addition to to the formians and thri-kreen mentioned above, consider faeries and pixies with butterfly wings or dragonfly wings, the cricket-legged grig (or whatever is was called), the human-faced larvae of the lower planes, the hook-horror and umber hulk. It seems that most of D&D insectoid monsters were not made open content however, and not many were player-oriented...

Dreamscarred Press updated the Expanded Psionics Handbook (Psionics Unleashed), but couldn't use the Thri-kreen because WoTC never made them Open Content.

So, they created the Dromites, which are insectoid.

You're right about the Thri-Kreen not being open, but Dromites were in the Expanded Psionics Handbook as well.

Dreamscarred has altered the Dromites a bit, though. XPH Dromites, as I recall, were especially humanoid, almost to the point of just being bug-eyed halflings. PsiU Dromites are more insectoid, but still just have four limbs.

Dromites are much more anthropomorphized than Kreen, but are still a good example of anthropomorphized insects.

I, being a bit broken in the mind by too many 1950's B movies, have included a race of "Bumble-Bee" people, in my campaign setting. They live in hives built underground, and you normally only see the females. The females resemble normal human-ish women, but have four arms, large multi-faceted eyes, and very big, ahem, hair.

Insect men are typically more common in sci-fi where their bug-like alien-ness makes them a perfect fit. You don't have to look any further than the Alien films to see anthropomorphized bug-men, and even the metaluna mutant from the classic This Island Earth is very insect-like with its segmented body and caliper-like claws. Either of The Fly movies (the original or the awesome Cronenberg remake) or even a few Spider-Man issues and villains are also good examples.

It was probably no coincidence the Spelljammer D&D setting had so many insect references. One race of humanoids, the Insectare, were basically just elves with antennae, but they shared a hive mind and a rigid, lawful society. Another less-humanoid race, the Xixchil, were a mantis-like race of surgeons and craftsmen who could be paid to apply beneficial mutations to people. On top of that, many spelljamming vessels were shaped like bugs. Nearly every Elf ship had a butterfly or moth shape to it and the alien Neogi slavers had ships shaped like spiders.

The thri-kreen and dromites follow the trend since psionics is more often associated with sci-fi than sorcery. None of this means insect-people can't be part of sword and sorcery fantasy. It's just more common in sci-fi because the nature and appearance of many insects is very alien to a lot of people so it's easier to imagine them as creatures from another world.

Spelljammer also had the...bionoids? i'm not 100% sure on the name, but they were humans (elves? helf elves? fuzzy memory) who could turn into a humanoid-insect-type form for fighting. (think the Guyver anime)

Recently I was reading a comic, in it the group of heroes could take some drugs to turn them into anthropomorphism of insect temporarily. It gave me some idea of building up a character that can turn into insect-humanoid to fight for a few rounds. When I looked up through the rules, I found most abilities were about anthropomorphism of animals such as beastmorphs / anthropomorphic animal.

Then, I start to think about the most (none scientific) fantasy works I've read. Animal-headed humanoids seem to be pretty common, human-headed animals are rare but still can be seen once for a while. Insect anthropomorphism, however, just seems to be extremely rare. The closest thing is drider but spiders aren't insect. There are some anthropomorphism of spiders and centipedes in classical literature like journey to the west but neither of them are insect as well.

Is it some kind of taboo to use it in none-scientific fantasy settings?

Yep, thri-kreen. You could also take the giant praying mantis and make it into "people".

I'm going to throw in a praying mantis transformation item "It's dinner time!". I truly like the film, the fly. That had a really weird birth scene.

On the front of my games, the pcs met a friendly cleric of maggots and rot, which transformed two of them into a maggot form with a high metabolism, allowing them to be fed filth and recuperate injuries extremely quickly. Full health over 8 hours. Become a whole new being for a time.